The gate meter consists of a spring-loaded, hinged flap mounted at right angles to the
direction of fluid flow in the fluid-carrying pipe. The flap is connected to a pointer
outside the pipe. The fluid flow deflects the flap and pointer and the flow rate is
indicated by a graduated scale behind the pointer. The major difficulty with such
devices is in preventing leaks at the hinge point. A variation on this principle is the
air-vane meter, which measures deflection of the flap by a potentiometer inside the pipe.
This is commonly used to measure airflow within automotive fuel-injection systems.
Another similar device is the target meter. This consists of a circular disc-shaped flap
in the pipe. Fluid flow rate is inferred from the force exerted on the disc measured by
strain gauges bonded to it. This meter is very useful for measuring the flow of dilute
slurries but it does not find wide application elsewhere as it has a relatively high cost.
Measurement uncertainty in all of these types of meter varies between 1% and 5%
according to cost and design of each instrument.